The present invention relates to a plug-flow operated fermenter for anaerobic fermentation of biogenic waste, with an inlet and an outlet, and with an agitator which consists of a shaft passing through the fermenter in its longitudinal direction, on which a multitude of agitator arms are arranged, wherein the shaft is mounted in the region of the fermenter ends.
The applicant is the worldwide leader in providing installations for producing biogas from biogenic waste. The basic construction of such installations is to be deduced from the European patent document EP-0'476'217-A. The installations operate according to a method according to the European Patent EP-B-621'336. The firstly mentioned patent document discloses a plug-flow operated, horizontal fermenter which is suitable for the anaerobic fermentation of biogenic waste. The fermenter is an elongated, horizontal tank with an inlet provided at one end, and with an outlet present at the oppositely lying end. The biogenic waste, reduced in size, is fed in at the inlet side and is inoculated with fermented material and/or press water from the processing. The material to be fermented is enriched with methane bacteria by way of this. The biogenic waste, amid controlled thorough mixing, is decomposed while forming biogas, and subsequently led through the outlet for aerobic decay after the exit.
The worldwide demand for installations of systems of the type mentioned above to have increasing capacities also leads to ever-larger fermenters being built. In order to render this possible, the fermenter tanks must be set up on location, wherein this may be effected either by way of a segmented joining-together into a steel tank, or, as is known from EP-770'675-A, the horizontal fermenter tank may be set up on location from concrete. Nowadays, horizontal fermenter tanks are realized with a total length of over 50 meters and a diameter of over 10 meters for increasing the capacity. In previous systems, the shaft of the agitator was designed as a solid steel shaft. This is possible without any problems with lengths below 20 meters. If greater lengths are required, then the intrinsic weight of the shafts leads to a sagging, which is a problem. The agitator indeed must not only thoroughly mix the biogenic waste in order to achieve a certain homogeneity, but one must simultaneously also ensure that heavy solid matter, in particular such as sand and stones, do not sediment at the bottom of the fermenter tank and as a result may no longer be carried away. Although the fermenter is operated in plug-flow, the through-flow is not capable of carrying away the sinking heavy substances, since the plug-flow movement only has a low flow speed. The throughput time of the biogenic waste through the fermenter from the inlet to the outlet, in fact, is several days. The agitator as a result of this, apart from thoroughly mixing, likewise contributes to conveying this heavy matter upwards from the base again, in order afterwards to be transported with the subsequent sinking movement in the plug-flow, in the direction of the fermenter outlet. Accordingly, the agitator consists of a shaft passing through the fermenter, with a multitude of agitator arms, which at their end distant to the shaft are provided with suitable blades.
With greatly dimensioned fermenters, the sagging of the shaft has led to the blades practically brushing along on the fermenter wall, and accordingly the occurrence of defects of the fermenter after a few years of operation. In the most obvious solution, the shaft is mounted by way of intermediate supports. This solution however has not proven itself suitable, since the supports upset the plug-flow operation.
Finally, a fermenter is known from DE-A-31'49'344, which comprises an agitator which is provided with tank-like agitator arms. These agitator arms are designed such that gas may be pumped into these amid the displacement of fluid components from these tank-like agitator arms, so that their buoyancy effects a rotational movement of the agitator, without a driven shaft being required.
It is therefore the object of the invention to provide a solution which avoids a sagging of the shaft, and rules out damage resulting from this.